Fuel cells are clean power generators with high-energy efficiency, and are projected to generate a significant amount of total energy consumed in the next three decades. Due to their high power density, quick dynamic response to power demand, low operating temperature, and silent operation, polymer electrolyte membrane (PEM) fuel cells are envisioned for use in both vehicle and residential applications. The projected commercialization of polymer electrolyte membrane fuel cells requires the availability of economic hydrogen sources.
Reforming of various hydrocarbons, such as methanol, natural gas, gasoline, and diesel for the production of hydrogen, is being considered for above-mentioned applications because it offers higher energy density than metal hydride, nano carbon tube, and other types of hydrogen carriers. For fuel cell powered automobiles, gasoline fuel is more likely to be the fuel of choice due to its existing infrastructure of distribution and supply. Likewise, diesel fuel is more favorable for military use and for auxiliary and stand-by power.
Steam reforming and partial oxidation are two major processes of reforming hydrocarbons to produce hydrogen (with main by-products of carbon dioxide and carbon monoxide and possibly methane as an undesired byproduct). Steam reforming is currently the most important natural gas conversion process to produce synthetic gas. It has been estimated that 76% of all hydrogen used comes from steam reforming of natural gas. Partial oxidation can be conducted in three different approaches: autothermal reforming, catalytic partial oxidation, and non-catalytic partial oxidation. Autothermal reforming is actually a hybrid of partial oxidation and steam reforming. Noncatalytic partial oxidation usually requires higher temperatures (over 1000° C.) for a complete conversion than autothermal reforming and partial oxidation. Compared with partial oxidation, steam reforming produces a product with higher hydrogen concentration, thus benefiting the operation of PEM fuel cells.
Nickel supported catalysts (for example Ni/Al2O3), presently used in the industry as steam reforming catalysts for hydrocarbons, are readily poisoned by very small amounts of sulfur compounds contained in hydrocarbons, such as natural gas, gasoline, and diesel. Poisoning the catalyst causes it to lose its activity. Sulfur removal is essential if steam reforming reaction is conducted with any sulfur bearing hydrocarbon fuel and a nickel supported catalyst. Typically, the sulfur levels need to be reduced to 0.2 ppm or lower. Carbon deposition on nickel catalyst is also particularly problematic when a heavy hydrocarbon fuel, such as gasoline or diesel, is reformed. The carbon deposition on the nickel catalyst degrades catalyst performance by decreasing its activity, selectivity, and durability.
The removal of sulfur compounds from hydrocarbons is usually accomplished by a high temperature hydro-desulfurization (HDS) process in which sulfur-containing hydrocarbon is mixed with a small quantity of hydrogen and passed over a hot bed of catalyst (such as cobalt and molybdenum oxides supported on alumina). Organic sulfur components are reduced to H2S and hydrocarbons, and subsequently the H2S produced can be removed by adsorption on a bed of ZnO adsorbent to a level below 0.5 ppm. The optimal temperature for most HDS process is between 350 and 400° C. This temperature range is also suitable for the adsorption of H2S on a ZnO bed. Thermal cracking of organic sulfur compounds to H2S at elevated temperature is another high temperature HDS process in which no hydrogen is required.
Low temperature desulfurization adsorption processes are also used for the removal of sulfur compounds. The process does not require hydrogen. The two most widely used adsorbents are activated carbon and molecular sieves. Activated carbon can be chemically impregnated to enhance the adsorption of H2S and it is acceptable for use in small-scale systems for relatively short time periods. In both cases raising the bed temperatures can reactivate the adsorbents. For larger systems, these methods are impractical because of the large quantities of adsorbent required and problems associated with its reactivation and with disposal of the desorbed sulfur. While the foregoing desulfurization processes are effective, they introduce additional steps into the process of reforming hydrocarbon fuel into hydrogen. These steps increase the complexity of the overall reforming process, thus resulting in a bulky system.
It is therefore an object of the invention to provide an effective catalyst for steam reforming of various hydrocarbons to produce hydrogen for the use of fuel cells.
There are several technical papers published pertaining to the catalytic performance of Ru catalyst during the steam reforming under the presence of sulfur components. Suzuki et al reported that a Ru/Al2O3 catalyst maintained its high activity for over 24 hours with 0.1-ppm sulfur in the feed, while the conversion decreased to 72.3% after 24 hours using commercial Ni catalyst. With 51-ppm sulfur and using Ru/Al2O3 catalyst, the conversion of hydrocarbon decreased to 60% after 25 hours. The performance of Ru/Al2O3 in the presence of 51-ppm sulfur was improved (decreased to 85.5% after 25 hours) with the addition of CeO2 into Al2O3.
It is also noted that bimetallic Pt—Ru alloy is used as the anode catalyst of polymer electrolyte membrane fuel cells. The Pt—Ru catalysts are usually supported on a high surface area carbon (Vulcan XC72 is one example) with an atomic ratio of 1:1 when it is used as the anode catalyst of PEM fuel cells. In this application, the catalyst is used to catalyze the electrochemical reaction of H2=2H++2e at a temperature of about 70–90° C.
With the increasing interest in fuel cells as sources for “clean” energy, there is a need for a catalyst capable of steam reforming various hydrocarbons. The preferred properties of the catalyst include coke resistance when it is used for steam reforming of such fuels as gasoline and diesel, sulfur tolerance when it is used for steam reforming of sulfur containing fuel, such as natural gas, gasoline, and diesel. A sulfur tolerant and coke resistance catalyst is particularly important for on-board reforming of hydrocarbons or portable applications where a highly compact design is required.